1. Field of the Invention
The present invention relates to ion implantation, and more especially, to low temperature ion implantation.
2. Background of the Related Art
It has been discovered that a relatively low substrate temperature during ion implantation may be advantageous for formation of shallow junctions, especially ultra-shallow junctions, which is more and more important for continued miniaturization of the semiconductor devices. It also may be useful for enhancing the yield of the ion implantation.
At the start of the conventional low temperature ion implantation, a substrate is moved from an outside environment, such as an atmosphere environment, into an implanter before an implant process is started. As shown in FIG. 1a, a cooling process (from tc to ti) is performed to cool the substrate temperature from about environment temperature (TR), such as about 15˜25° C., to about a prescribed implant temperature (TP), such as about −15˜25° C., which usually is lower than the freezing point of water and is the e-chuck temperature during the implant process Herein, the substrate can be cooled at least in a cassette outside the implanter, in a load-lock of the implanter, in a chamber of the implanter, and so on.
In general, a backside gas is applied to cool the substrate and it requires several seconds (even several minutes) to cool down the substrate. Referring still to the FIG. 1a, during the implant process (from ti to th), the substrate is heated by the ion beam energy and cooled by a cooling mechanism, such as a backside cooling gas. Usually, to ensure the implantation quality on the substrate during the implant process, the operation of the cooling mechanism is properly adjusted to ensure the substrate temperature is always essentially equal to the prescribed implant temperature or at least is not higher than an upper-limited temperature (TL) during the implant process (from ti to th) Herein, the rise curve of the substrate temperature may be linear or non-linear. The rise curve during the implant process (from ti to th) shown in FIG. 1a is only a sketch. On the other hand, if the upper-limited temperature is close to the prescribed implant temperature, as shown in FIG. 1b, the rise curve during the implant process (from ti to th) shown in FIG. 1b may be a simplified as a horizontal straight line.
After finishing the implant process, referring still to FIG. 1a or FIG. 1b, a heating process (from th to tf) is performed to heat the implanted substrate to a quasi-environment temperature (TR′), and then the implanted substrate is moved out the implanter to the outside environment for subsequent semiconductor fabrication. Herein, the quasi-environment temperature may be close to a temperature of the atmosphere environment or higher than a dew point of water in the outside environment. Hence, the water condensation problem on the substrate surface induced by the temperature difference may be avoidable.
In the foregoing processes, it requires several seconds (even some minutes) to cool down the substrate from the environment temperature to the prescribed implant temperature. Also, it requires several seconds (even some minutes) to heat up the implanted substrate from the prescribed implant temperature to the quasi-environment temperature. Moreover, to ensure the uniformity and quality of the implant process, both the cooling process and the heating process are separated from the implant process during the low temperature ion implantation. However, both the cooling process and heating process are time-consuming, so that the throughput of the low temperature ion implantation is limited.
Accordingly, there is a need to propose a novel and effective approach to improve the low temperature ion implantation.